1. Field of the Invention
The subject disclosure relates to fuel cells, and more particularly to solid oxide fuel cell systems having hot zones with improved reactant distribution, more uniform heat distribution, and/or more efficient heat retention.
2. Background of the Related Art
Referring to FIG. 1, a schematic view of a generator 110 with a hot zone 112 is shown. The generator 110 has an electrochemical stack or solid oxide fuel cell (SOFC) 120 typically operating at temperatures above 600° C. Several other support components are also operating at elevated temperature.
Commonly, the SOFC 120 is located within a hot environment to facilitate proper operation. In some instances, an external furnace could provide a hot environment but the furnace size, weight, and power consumption may negate most practical benefits in many applications. For example, for practical portable power generation applications, the SOFC 120 is preferably able to maintain stack temperature and operating environment in a compact and efficient package.
To accomplish maintaining the endothermic components at sufficient temperature, the SOFC 120 has an area of integration of the elevated temperature components, referred to as the hot zone 112. The hot zone 112 is insulated to reduce heat loss and maintain the desired operating temperature. The components of the hot zone 112 include a fuel processing reactor such as, without limitation, a catalytic partial oxidation (CPOX) reactor 114 for converting the system feed hydrocarbon fuel to a hydrogen and carbon monoxide rich feed for the stack 120 of the generator 110. For example, the CPOX reactor may convert a hydrocarbon fuel like propane or diesel to hydrogen and carbon monoxide by reaction with air over a catalyst. The CPOX product gases are then fed to the anode side of the SOFC stack 120. For liquid fuels, the feed hydrocarbon fuel is readied for the CPOX reactor 114 by using an atomizer or vaporizer.
The hot zone 112 also includes an exothermic tail gas combustor 116 that burns the remaining unutilized fuel from the stack 120 to reduce CO emissions and also to utilize the remaining fuel energy in the form of heat within the hot zone 112. For example, some hydrogen and carbon monoxide are oxidized with oxygen that is transported across the SOFC electrolyte (not shown explicitly) from the air on the cathode side of the SOFC stack 120. This remaining hydrogen and carbon monoxide are mixed with cathode exhaust in the tail gas combustor 116 for burning in a homogenous flame and/or over a catalyst.
The tail gas combustor exhaust enters a recuperator 118 where the gas is cooled by exchanging heat with the incoming cathode air. The recuperator heat exchanger 118 maintains heat within the hot zone 112 by transferring heat from the SOFC exhaust gas to the inlet stack air.
A power conditioning unit 124 also connects to the electrochemical stack 120. A CPOX air blower 126 provides air to the CPOX reactor 114. A fuel pump 130 expels fuel from a fuel tank 128 to the vaporizer 122. A cathode blower 132 provides air to the recuperator 118 and cathode side of the electrochemical stack 120.
The approach of FIG. 1 has several disadvantages. Each component requires piping to connect to the neighboring component. This plumbing requires a high temperature sealing method such as brazing or welding, a very labor intensive and extremely difficult to automate process. Each hot zone component also requires special features for braze or weld joints to the attached piping. These special features are typically machined, and result in high cost components.
Further, some stack components are ceramic, and sealing metal-to-ceramic joints is difficult such as shown in U.S. Patent Application Publication No. 2004/0195782 published on Oct. 7, 2004. The hot CPOX reactor 114 and tail gas combustor 116 are located away from the electrochemical stack 120, which slows heating at startup. Assembling and joining these components in close proximity is difficult and requires packing a large volume in a small space. Consequently, the support components can undesirably occupy as much hot zone volume as the electrochemical stack 120. The inability to closely integrate the hot zone components leads to a low hot zone power density and uneven distribution of heat. Further, the large insulation surface around the hot zone 112 may increase heat loss.
High fuel utilization is particularly desirable for high generator efficiency. The generator 110 cannot alone utilize the fuel unless the SOFC 120 is impracticably large. Thus, the performance of the tail gas combustor 116 is particularly important and serves as a source of thermal energy that can be used for other needs. For instance, the thermal energy can keep the stack 120 at operating temperature and balance heat losses through insulation and system exhaust.
U.S. patent application Ser. No. 12/006,688, filed on Jan. 4, 2008 (the '688 application), also recognized many shortcomings of the prior art and provided improvements thereto. The '688 application disclosed a solid oxide fuel cell system including a main plate, an inner cylinder attached to the main plate, an intermediate cylinder attached to the main plate such that the intermediate cylinder contains a cathode air stream, and an outer cylinder attached to the main plate. An exhaust annular gap is formed between the intermediate and outer cylinders such that hot exhaust gases co-flow through the exhaust annular gap and heat is transferred from the hot exhaust gases to the cathode air stream.
The hot zone 112 provides key functions to support the SOFC stack operation including even temperature distribution throughout the stack volume, even flow distribution of fuel gases to the anodes of all cells in the stack and even flow distribution of air to the cathodes of all cells in the stack. Combustion of unutilized fuel in the stack exhaust may also occur in the hot zone. Preferably, there are minimal thermal losses through conduction across the hot zone boundaries and minimal thermal losses through hot exhaust gases.
Another function of the hot zone 112 is to maintain stack operating temperature within an appropriate range. Stack operating temperature must be high enough to maximize electrolyte ionic conductivity and prevent carbon formation. Stack temperature must not be too high, however, to avoid lowering the open circuit potential, increasing the electrolyte electrical conductivity, and initiating thermal degradation of cell or system materials. These temperature limits depend on the materials used in the cells and stack, and an exemplary ideal temperature range might be between 700 and 800° C. Keeping all cells in the stack 120 within such a temperature range requires a design that minimizes thermal gradients in the stack.
U.S. Pat. No. 6,492,050, issued on Dec. 10, 2002 to Sammes, attempted to provide an integrated solid oxide fuel cell and reformer. However, the heat exchanger design of Sammes has little surface area, which leads to poor performance. Also the Sammes system is thermally unbalanced not only axially but radial temperature variation is also significant with regions of stagnant air.